vhost_user_fs: Add the WRITE_KILL_PRIV write flag.

Add the WRITE_KILL_PRIV write flag, corresponding to
FUSE_WRITE_KILL_PRIV introduced in 7.31, and use to only remove the
setuid and setgid bits (by switching credentials) conditionally.

Signed-off-by: Sergio Lopez <slp@redhat.com>

vhost_user_fs/src/server.rs

@@ -636,6 +636,7 @@ impl<F: FileSystem + Sync> Server<F> {
         };
 
         let delayed_write = write_flags & WRITE_CACHE != 0;
+        let kill_priv = write_flags & WRITE_KILL_PRIV != 0;
 
         let data_reader = ZCReader(r);
 
@@ -648,6 +649,7 @@ impl<F: FileSystem + Sync> Server<F> {
             offset,
             owner,
             delayed_write,
+            kill_priv,
             flags,
         ) {
             Ok(count) => {